But the thousands of Shiite fighters – some sporting suicide vests and calling themselves the “Peace Brigades” - also sent another signal: feared firebrand cleric Muqtada al-Sadr and his Mahdi Army are back.

While Sadr was once the poster boy for violent resistance to the U.S.-led invasion, he has in recent years positioned himself as a political kingmaker in post-withdrawal Iraq and even publicly discouraged sectarian strife.

But analysts say that his recent calls for brigades to form and protect holy shrines from attacks by Sunni militants – which culminated in the weekend’s show of force - betray a canny commandant and risk returning Iraq to some of its darkest days.

"The forces that he puts on the streets are some of the worst sectarian killers in Iraq"

“It means two things - that Iraqi security forces are incapable of providing security in Baghdad and the chance of the re-emergence of sectarian slaughter - with death squads roaming Baghdad and other areas around the capital - increases dramatically,” said Bill Roggio, editor of The Long War Journal.

Roggio - like many - question Sadr's attempts to play a moderate card.

“He speaks a good game but watch what Sadr does, not what he says. That’s been the real problem with him all along,” he added. “He may speak a good game, but the forces that he puts on the streets are some of the worst sectarian killers in Iraq.”

Sadr formed the Mahdi Army in 2003 after the fall of Saddam Hussein’s regime. The Shiite militia gained broader notoriety when it waged fierce battles in Najaf against U.S. forces a year later. It is blamed for the mass killings of Sunni civilians in sectarian violence in Iraq that peaked in 2006 and 2007 and for cleansing Sunnis from Baghdad neighborhoods.

The Pentagon once said the Mahdi Army had “replaced al Qaeda in Iraq as the most dangerous accelerant of potentially self-sustaining sectarian violence.”

Shiite cleric Muqtada al-Sadr shows his ink-stained finger after voting in parliamentary election at a polling station in Najaf, south of Baghdad on April 30, 2014.Alaa Al-Marjani / Reuters

When Sadr ordered a ceasefire in 2007, the move was accompanied by questions over whether he sought to wait out U.S. forces or to make a political play.

Sadr helped usher Iraq’s Shiite Prime Minister Nouri al-Maliki to power in 2006 and ultimately again in 2010 – but since then has become one of the premier’s most vocal critics, describing him as a dictator.

In February, Sadr said he was withdrawing from all politics and unaffiliated with any faction. His explanation: a necessary decision to protect the family name.

The puzzling move – accompanied by fierce criticism of Maliki’s government - prompted rampant speculation that the cleric was repositioning himself – or reinventing his brigade – rather than retiring.

Few “really bought it” when Sadr said in Feburary he was stepping away from politics, according to independent Iraq analyst Stephen Wicken,

“He’s much more canny than that,” Wicken said. “He plays this reluctant hero card: that he doesn’t want to be dragged into politics and public life but then has to by force of his belief or weight of his family’s legacy.”

Those guesses appeared to play out when Sadr recently called for brigades to form and protect Shiite shrines from the lightning advance and offensive by Sunni extremist fighters from the Islamic State of Iraq and al-Sham (ISIS).

The heavily armed fighters who answered his call marched through Iraq’s cities over the weekend under the name “Saraya Al Salaam” – or Peace Brigades – but to analysts and Iraq watchers, they are the officially inactive Mahdi Army - rebranded.

Sadr’s ability to call out massive numbers of supporters at a moment’s notice has been one of the cleric’s “hallmarks” over the years, according to Wicken. In this case, calling up fighters in the name of protecting Shiite interests could present a dig at Maliki, whose forces have been unable to stave of the ISIS advance. “This is an opportunity for Sadr,” Wicken said.

"If the state needs to rely on thugs like Sadr and others to secure areas that are being threatened by ISIS ... then it is really in trouble"

While Sadr’s ultimate ambition might be to achieve the spiritual authority of his forebears - both his father and father-in-law were Grand Ayatollahs - “he doesn’t actually have the personality” or gravitas to become a religious authority, according to Wicken.

Sadr’s timing is everything, according to analysts, who point to the cleric’s tendency to step away from day-to-day politics and return when opportunity is ripe to reclaim the mantle of Shiite defender.

“He’s not a serious cleric, he’s not a religious authority. The only power and legitimacy that he actually has comes primarily from his family’s name and legacy and also from his willingness to take really populist platforms and, when necessary, call out his forces to kill people,” Wicken said.

Sadr’s real credentials stem from his role in founding and commanding the Mahdi Army. While he has lost some ground to splinter groups in the past few years, “he still has that resonance,” Wicken said. While Sadr is not a “gifted orator, his words carry weight,” according to Wicken.

Putting fighters on the streets would signal to Sadr's Shiite rivals that “he’s still relevant,” according to Hayder al-Khoei, an Iraq analyst for London-based think tank Chatham House.

“He’s played his hand now,” al-Khoei said. “He’s back and he’s back for good.”

Rebranding the militia as the Peace Brigades reflects a broader attempt to put distance between atrocities commited by Sadr’s fighters against Sunnis in 2006 and 2007.

“They’re trying to shed that black history that they have,” al-Khoei added. “Lots of Sunni Iraqis remember the role they played."

The mobilization of Sadr's militia adds to the polarization and portrayal of Iraq’s current crisis as a Shiite-Sunni conflict, analysts say.

Still, the cleric's battle-hardened and ideologically driven fighters could be an asset to Iraq’s security forces if they are deployed for operations.

"They can add experience in the counterinsurgency campaign - experience that the Iraqi army is desperate for,” al-Khoei said.

While “Sadr has always been more than willing to use force to get what he wants in Iraq,” Roggio said the cleric's recent re-entry also underscores a deeper point.

“Its further evidence of the deterioration of the state,” Roggio said. “If the state needs to rely on thugs like Sadr and others to secure areas that are being threatened by ISIS and regain control of areas outside of Baghdad, then it is really in trouble. Once the sectarian groups like the Mahdi Army reach the streets, it just greatly increases the chances of the sectarian killing that brought Iraq to the brink of full-scale civil war."